The invention relates generally to moisture detecting systems and more particularly to instrumentation for measuring trace moisture in a sample fluid stream.
The need arises in connection with chemical processing, for example, at refineries, to keep track of extremely small percentages of water or water vapor present in a fluid stream. In certain applications continuous accurate measurement of trace moisture in fluids is necessary to meet product and corrosion specifications. For example, accurate control of the moisture level in hydrogen recycle gas is necessary to assure optimum catalyst life.
Process moisture analyzers are known in which a sample gaseous stream is placed in contact with a dielectric desiccant in an electrical capacitance cell. The capacitance varies as a function of the amount of water in the desiccant. To measure water in solids, another type of moisture analyzer, based on an entirely different principle, makes use of the ability of hydrogen nuclei, which are abundant in water, to slow down fast neutrons to thermal energies. Wood chips, paper and other bulk material are analyzed by this technique in which a neutron source irradiates material carried on a conveyor belt. A detector located beneath the conveyor senses slow neutrons indicative of water. This type of gauge would not be suitable for trace moisture detection because of the low incidence of water molecules. Nor could it differentiate hydrogen gas, for example, from water vapor.
The prior art capacitance cell technique for trace moisture detection in a sample stream suffers from the disadvantage that the sensitivity of the dielectric constant to the presence of water is easily affected by impurities.
One of the specific applications for trace moisture analysis is gaseous streams of hydrocarbons or hot hydrogen gases containing a trace of water vapor. Because of the critical effect of water vapor in the processes involving gaseous streams of this kind, the unreliability of the capacitance cell instrument is a major drawback.